Open drinking water reservoirs have been often-beloved icons in the United States for well over a century. Highland Park Reservoir (1879), McMillan Reservoir (1903), and Silver Lake Reservoir (1907), among others, were the places to promenade, picnic, see, and be seen in Pittsburgh, Washington, D.C., and Los Angeles, respectively.

Cool Spring Reservoir in Wilmington, Del. before it was buried. Credit: Philip Franks, Hurley-Franks and Associates.

Some, like Baltimore’s Druid Hill Park Reservoir, were located within larger park spaces; others, like Compton Hill Reservoir in St. Louis, essentially filled the entire space of their own park-like setting. It was recognized that none of them was entirely hygienic. They were fenced but, after all, at the mercy of general city dust and grime, not to mention bird droppings. But, like Ivory soap in the old commercial, 99.44 percent pure was considered good enough.

There are also numerous reservoirs that are not fenced. These reservoirs contain what is called “raw” water that is relatively clean but not yet “finished” for human consumption. At Griggs Reservoir Park in Columbus, Ohio, or White Rock Lake Park in Dallas visitors can go right to the water’s edge and dip their toes in, if they wish, or even go boating.

Then in 1993 came a highly publicized outbreak of Cryptosporidium bacteria in the Milwaukee water supply, and, soon after, heightened concerns about terrorism. Attention to public health was raised a notch. In December, 2005, after years of deliberation, the U.S. Environmental Protection Agency published something called the Long Term 2 Enhanced Surface Water Treatment Rule (LT2) that mandated that all newly constructed “finished water” reservoirs be built with a cover. (Finished water is clean enough for delivery to homes; raw water still needs treatment before it’s drinkable.) As for already existing finished water reservoirs, EPA gave municipalities the choice of covering them or leaving them as-is and then re-treating the water to finish it.

The requirement was greeted with dismay by many people who delight in the view of the open water, but the presence of a cover opens up the possibility for gaining parkland. Seattle, in particular, has recognized this chance to close a park gap in some neighborhoods. In fact, the city (along with the whole state of Washington) got started more than a decade ahead of the EPA rule. As former Mayor Greg Nickels put it, “This is a rare opportunity to turn public works into public parks. Underground reservoirs will not only improve the quality and security of our water supply, they will add to the quality of life in our neighborhoods.” All in all, the city is set to add 76 acres of new parkland using reservoir decks – including 4 acres in densely populated Capitol Hill, 20 acres in Jefferson Park (with a running track, sports fields, picnic grounds and a large, unprogrammed lawn), and a completely new park on top of Myrtle Reservoir. The $161-million cost is being funded via a rise in residential water use fees.

Cool Spring Reservoir in Wilmington, Del. after it was buried in a concrete tank. Note the grassy field and ornamental pond. Photo taken one month before opening day. Credit: Rory MacRory, AECOM.

Wilmington, Delaware, is getting a significant parkland boost from a similar program. Cool Spring Reservoir, which dates to 1875 and is located in a densely populated section, was buried in 2009, adding about 7 acres of parkland to the adjoining 12.5-acre Cool Spring Park. In one swoop, this conversion increased the small city’s total parkland resource by 1.6 percent. The expanded park serves about 11,500 residents within a half-mile radius.

Under the EPA rule, cities have the option of covering their finished-water reservoirs with a variety of materials, from air-supported fabric to floating polypropylene, from a dome of aluminum to a flat surface of wood, steel, or concrete. An analysis of possibilities for 15-acre Elysian Reservoir by the Los Angeles Department of Water and Power pegged the cost of a floating cover at $19.6 million, a lightweight aluminum roof at $38.1 million, and a buried concrete tank at $121.4 million. Seattle, of course, found the same type of steep costs, but the mayor’s office there conducted a study that showed acquiring a similar amount of other parkland would cost about 85 percent as much as putting the reservoirs in concrete tanks. Michael Shiosaki, Seattle’s deputy director of planning said, “There’s no way we’d be able to buy properties like this, situated as they are on scenic overlooks in densely built-out locations throughout the city.” The concrete decks are covered with 8 inches to 2 feet of soil and planted with grass. They are principally used as open lawn areas, active sports fields, and game courts, interwoven with pathways. Trees are restricted to the perimeter because of the risk of root penetration of the deck.

The tension of shimmering views versus safe drinking water is not new and it’s not unsolvable. St. Louis long ago figured out how to do it: For more than 100 years, Compton Hill Reservoir has been covered, but the top of the cover is bowl-shaped and filled with water – non-drinking water – to make for a beautiful park experience. Seattle did something similar, building a small non-drinking water pond and fountain on top of its new Cal Anderson Park deck to memorialize the former reservoir. Wilmington also responded to a neighborhood outcry, putting its reservoir under just half the property and redesigning the other half as a pond with a viewing platform.

Not all reservoir stories have happy outcomes. Washington, D.C.’s McMillan Reservoir, built in the early 1900s and envisioned as a central feature in the city’s open space network, has been closed to the public since World War II. The grounds of the reservoir and its associated sand filtration site total 118 acres in a part of the city with little other usable parkland. Originally designed in 1907 by Frederick Law Olmsted, Jr. as a public park with promenades and places for people to sit, the facility is today encircled by a rusty chain-link fence set far back from the water pool itself, precluding any human use of the grounds. Ironically, since the water is unfinished the EPA rule does not come into play and there is no mandate to cover or bury it. The managing agency, the U.S. Army Corps of Engineers, is concerned about possible water contamination and has no plans to remove or move the fence to get better use of the surrounding green space, and the neighborhood is not powerful or well-organized enough to push the Corps to think more creatively.

We’ve written before how reservoirs can be used as city parks, with some photos of the famed Cal Anderson Park. Additional Seattle reservoirs converted to parks with their opening dates include:

We’ve also written about an international park-to-reservoir, Padding Reservoir Gardens in Sydney. This historic reservoir is unique in that the underground ruins were preserved and kept publicly accessible.